Midwestern Oak Forest

This system is found throughout the glaciated regions of the Midwest, typically in gently rolling landscapes. It can occur on uplands within the prairie matrix and near floodplains, or on rolling glacial moraines and among kettle-kame topography. Soils are typically well-drained Mollisols or Alfisols that range from loamy to sandy loam or even coarse sands in texture. Historically, this type was quite extensive in Michigan, Indiana, Illinois, Missouri, Iowa, Wisconsin, and Minnesota. Well over 700,000 hectares likely occurred in southern Michigan alone (ca. 1800). It is distinguished from other forested systems within the region by a dry-mesic edaphic condition that is transitional between dry oak forests and woodlands and mesic hardwood forests, such as maple-basswood forests. Forest cover can range from a dense to moderately open canopy and there is commonly a dense shrub layer. Fire-resistant oak species, in particular Quercus macrocarpa, Quercus rubra, and/or Quercus alba, dominate the overstory. Carya spp., including Carya ovata, Carya cordiformis, and Carya tomentosa, are diagnostic in portions of the range of this system. Depending on site location and overstory canopy density, the understory may include species such as Amelanchier spp., Aralia nudicaulis, Corylus americana, Desmodium glutinosum, Maianthemum stellatum, Osmunda claytoniana, Phryma leptostachya, Trillium grandiflorum, and Viburnum acerifolium. Occasionally, prairie grasses such as Andropogon gerardii and Panicum virgatum may be present. Fire constitutes the main natural process for this type and likely maintained a more open canopy structure to support oak regeneration. Historic fire frequency was likely highest in the prairie-forest border areas. Fire suppression may account for the more closed oak forest examples of this system with the more mesic understory. It likely has allowed for other associates, such as Acer saccharum, Acer rubrum, Celtis occidentalis, Liriodendron tulipifera, Ostrya virginiana, and Juglans nigra, to become more prevalent, especially in upland areas along floodplains. Periodic drought, intensified by local conditions, such as slope, southern exposure, or sandy soil, also inhibit growth of mesophytic trees. Extensive conversion for agriculture has fragmented this system. Continued fire suppression has also resulted in succession to mesic hardwoods, such that in many locations, no oak species are regenerating. Remaining large areas of this system are likely under considerable pressure due to conversion to agriculture, pastureland, and urban development.

Forest cover can range from a dense to moderately open canopy and there is commonly a dense shrub layer. Fire-resistant oak species, in particular Quercus macrocarpa, Quercus rubra, and/or Quercus alba, dominate the overstory. Carya spp., including Carya ovata, Carya cordiformis, and Carya tomentosa, are diagnostic in portions of the range of this system. Depending on site location and overstory canopy density, the understory may include species such as Corylus americana, Amelanchier spp., Maianthemum stellatum, Caulophyllum thalictroides, Laportea canadensis, Trillium grandiflorum, Aralia nudicaulis, and Urtica dioica. Occasionally, prairie grasses such as Andropogon gerardii and Panicum virgatum may be present. Fire suppression likely has allowed for other associates, such as Acer saccharum, Celtis occidentalis, Liriodendron tulipifera, Ostrya virginiana, and Juglans nigra, to become more prevalent, especially in upland areas along floodplains.

This system can occur on uplands within the prairie matrix and near floodplains, or on rolling glacial moraines and kettle-kame topography. Soils are typically well-drained Mollisols or Alfisols that range from loamy to sandy loam or even coarse sands in texture. Historically, this type was quite extensive in Michigan, Indiana, Illinois, Missouri, Iowa, Wisconsin, and Minnesota. Well over 700,000 hectares likely occurred in southern Michigan alone (ca. 1800). It is distinguished from other forested systems within the region by a dry-mesic edaphic condition that is transitional between dry oak forests and woodlands and mesic hardwood forests, such as maple-basswood forests.

Fire constitutes the main natural process for this type and frequent surface fires combined with uncommon crown fires maintained a more open canopy and subcanopy structure to allow oak regeneration. Historic fire frequency was highest in the prairie-forest border areas and declined further from prairies and behind natural firebreaks. Frequency of fires necessary to maintain this system varied, largely depending on soil fertility and drainage, with more fertile and mesic sites requiring more frequent fires. Fire-return intervals of 15-25 years would typically maintain a woodland, while fire-return intervals of 25-50 years would typically maintain a closed-canopy oak forest (Landfire 2007a). Fire suppression accounts for many of the more closed oak forest examples of this system with the more mesic understory (Abrams 1992, Lorimer 2001). Fire suppression has allowed for other associates, such as Acer saccharum, Celtis occidentalis, Juglans nigra, Liriodendron tulipifera, Ostrya virginiana, and invasive shrubs, to become more prevalent, especially in more mesic upland areas or along floodplains (Rogers et al. 2008). Periodic drought, intensified by local conditions like slope, southern exposure, or sandy soil, also inhibit growth of mesophytic trees. Some stands currently in this system were more open savanna stands but fire suppression has allowed them to succeed to the more close-canopy oak woodland or forest. A continued lack of fire in many of those stands will result in succession to more mesophytic forest types. Gap-phase dynamics producing multi-structured, uneven-aged stands operate most noticeably in North-Central Interior Beech-Maple Forest (CES202.693) but also influence succession in this system. Canopy gap formation originates through localized stem breakage resulting from wind (Runkle 1982), glaze or ice storms (Lemon 1961), attack by oak wilt fungus (Chalara quercina), and episodic defoliation caused by insects such as gypsy moth (Lymantria dispar).

Fire suppression, logging, and conversion to agricultural or urban uses are the major threats to this system. Fire suppression has resulted in more closed-canopy forests with little or no oak regeneration and invasion by fire-intolerant trees, such as Acer spp., Celtis occidentalis, Fagus grandifolia, and Liriodendron tulipifera, and understory species (Nowacki and Abrams 2008), especially invasive shrubs. Extensive conversion for agriculture has fragmented this system and the landscape in which it exists and the resulting increase in edge-to-interior ratio has encouraged invasion by weedy species. Even where the tree canopy is maintained in a moderately closed condition, invasion by exotic shrubs, particularly Rhamnus cathartica and Lonicera spp., can form nearly continuous shrub canopies in some stands, shading out regeneration of all but the most shade-tolerant species. Stands can be significantly affected by insect defoliators Spongy Moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria). Long-term effects from these have been limited in this system, but significant mortality in Quercus-dominated stands in the northeastern U.S. has been noted due to Spongy Moth defoliation (Davidson et al. 2001). Deer browsing can affect the understory and tree seedlings, reducing many native forbs and tree seedlings (Healy 1997, Rooney 2001). White-tailed deer have been shown to browse oak seedling over maple seedlings (Stroke and Anderson 1992). Grazing by livestock can have impacts on sensitive understory species. Impacts of logging can include shift in species composition (especially where overstory species are high-graded) and decrease in species diversity.

This system is found throughout the glaciated regions of the Midwest south of the tension zone.